First-principles investigation of BiFeO3 and related multiferroic materials.

Abstract

This work is about magnetoeltric multiferroics, a relatively new class of ma-_x000D_ terials discovered by the mid of the past century, which involve simultaneously_x000D_ ferroelectricity and magnetism. Perovskite oxide BiFeO3 (BFO) is one of the few_x000D_ multiferroic materials at room temperature. However, as its ferroelectric and anti-_x000D_ ferromagnetic transition temperatures are relatively high (about 1100 K and 640_x000D_ K, respectively), BFO's electromechanical and magnetoelectric responses are small_x000D_ at ambient conditions._x000D_ In this thesis we used ab-initio methods, based on density functional theory, to_x000D_ study the basic properties of BFO and proposed possible strategies for enhancing_x000D_ its response. We used rst-principles methods to perform a systematic search for_x000D_ potentially stable phases of BFO. We considered the distortions that are most_x000D_ common among perovskite oxides and found a large number of local minima of the_x000D_ energy. We discussed the variety of low-symmetry structures discovered, as well_x000D_ as the implications of these ndings as regards current experimental work on this_x000D_ compound._x000D_ We also carried out a study of the Bi1􀀀�xLaxFeO3 (BLFO) solid solution formed_x000D_ by multiferroic BFO and the paraelectric antiferromagnet LaFeO3 (LFO). We dis-_x000D_ cussed the structural transformations that BLFO undergoes as a function of La_x000D_ content and the connection of our results with the existing crystallographic stud-_x000D_ ies. We found that, in a wide range of intermediate compositions, BLFO presents_x000D_ competitive phases that are essentially degenerate in energy. Further, our results_x000D_ suggested that, within this unusual morphotropic region, an electric eld might_x000D_ be used to induce various types of paraelectric-to-ferroelectric transitions in the_x000D_ compound. We also discussed BLFO's response properties and showed that they_x000D_ can be signi cantly enhanced by partial substitution of Bi/La atoms in the pure_x000D_ BFO and LFO materials. We analyzed the atomistic mechanisms responsible for_x000D_ such improved properties and showed that the e ects can be captured by simple_x000D_ phenomenological models that treat explicitly the composition x in a Landau-like_x000D_ potential._x000D_ Furthermore, we performed a rst-principles study of BFO at high pressures. Our_x000D_ work revealed the main structural change in Bi's coordination and suppression of_x000D_ the ferroelectric distortion, electronic spin crossover and metallization, and mag-_x000D_ netic loss of order e ects favored by compression and how they are connected. Our_x000D_ results are consistent with and explain the striking manifold transitions observed_x000D_ experimentally_x000D_ We conclude our thesis presenting the preliminary results of an ongoing project_x000D_ in which we are modeling the energetics of the oxygen octahedra rotations in per-_x000D_ ovskite oxides. The model is tted to the rst-principles results and a careful check_x000D_ of its validity is carried out.

Date of Award	6 Jul 2012
Original language	English
Supervisor	Jorge Íñiguez González (Director)